Passive RFID tags obtain their operational power from radio frequency (RF) energy provided by the tag reader. As a result, passive RFID tags are susceptible to power loss caused by conditions in their operating environment. For example, reflections of transmitted signals off surfaces in the operating location may cause multipath cancellation. This multipath cancellation causes RF nulls to be created at various points in the operating location. An RF null is a location where the signal drops-out (e.g., the signal strength is essentially zero). Thus, a passive RFID tag may loose power and regain power when moving through an RF null. In addition, the use of frequency hopping procedures may also cause a tag to loose and regain power. Currently, the Federal Communication Commission (FCC) requires a reader to change frequency of operation on a periodic basis ranging from 50 to 400 milliseconds. During this period of time, the reader adjusts its frequency to a new frequency. The change in reader frequency may create rapid changes in RF energy. This rapid change in RF energy may in turn force tags into and out of RF nulls.
In a conventional passive RFID tag, the tag is unable to maintain its state after a loss of power. When power is restored to the tag, the tag is reset. Thus, the tag may enter a state indicating it has not been read. As a result, a tag may be re-negotiated by the reader even though the tag has already been successfully read. These superfluous negotiations of tags introduce inefficiency into the tag interrogation process. As the size of the tag population increases, the inefficiency introduced by unwanted re-negotiation of read tags also increases.
One technique to address the inefficiency introduced by the loss of tag state is to implement a persistent state on the tag. For example, a transient memory device such as a capacitor can be used to hold the state of a tag during power loss. However, in this technique, the length of retention of the value in memory can vary from less than a second to a half-second or more. This variation is due to the fact that voltage drains from the capacitor over time. Furthermore, voltage amplitude can vary based on distance from the reader, also introducing variations in the length of transient storage. In addition, temperature often widely varies the characteristics of a silicon storage device which greatly adds to the operational range in length of retention.
In addition, using this technique, the reader has no control over the memory device in each tag. A tag retains knowledge of its persistent state until the memory device times-out. Therefore, a reader cannot re-negotiate all tags in a population of tags until the persistent “read” state stored in each tag has timed out.
Consequently, a need exists for reader controlled maintenance and management of persistent states stored in passive RFID tags.